1. Field of the Invention
The invention relates to a control apparatus of an internal combustion engine mounted in a vehicle. More particularly, the invention relates to improving control for suppressing vibration in an internal combustion engine that is provided with a variable valve timing mechanism.
2. Description of the Related Art
In an internal combustion engine (hereinafter also referred to simply as an “engine”) mounted in a vehicle or the like, rotation of a crankshaft is transmitted to a camshaft via a timing belt or the like. Valves (i.e., intake valves and exhaust valves) provided in a valve system of the engine move up and down by being cyclically pushed down by cams on the camshaft, thereby opening and closing the intake and exhaust passages.
Also, one known valve system of an engine is equipped with a variable valve timing (VVT) mechanism to improve engine output, the fuel consumption rate, and emissions and the like is known (see Japanese Patent Application Publication No. 2006-138292 (JP-A-2006-138292) and Japanese Patent Application Publication No. 2004-332662 (JP-A-2004-332662)).
This variable valve timing mechanism changes the valve opening/closing timing by changing the rotation phase of the camshaft with respect to the crankshaft. One generally known variable valve timing mechanism is a vane type variable valve timing mechanism. This vane type variable valve timing mechanism includes, for example, a housing with concave portions formed on the inner peripheral surface, and an internal rotor that has vanes that divide the concave portions of the housing into two hydraulic chambers (i.e., a retard side hydraulic chamber and an advance side hydraulic chamber) each. Also, the vane type variable valve timing mechanism is structured such that the housing receives rotative force from the crankshaft via a timing belt or the like, and the internal rotor is connected to the camshaft. The valve opening/closing timing is continuously changed by changing the rotation phase of the crankshaft and the camshaft, which is done by controlling the hydraulic pressure supplied to the retard side hydraulic chamber and the advance side hydraulic chamber with an oil control valve (OCV).
The oil control valve is formed by a spool arranged so as to be able to move back and forth inside a casing, a compression coil spring that applies urging force to the spool, and an electromagnetic solenoid that attracts the spool when voltage is applied, and the like. The voltage applied to the electromagnetic solenoid is duty controlled. The attraction force generated by the electromagnetic solenoid changes according to the duty ratio of the applied voltage. As the duty ratio of the voltage applied to the electromagnetic solenoid increases, the amount of hydraulic pressure supplied to the advance side hydraulic chamber increases, and as that duty ratio decreases, the amount of hydraulic pressure supplied to the retard side hydraulic chamber increases. In this way, the variable valve timing mechanism is driven by adjusting the hydraulic pressures in the advance side hydraulic chamber and the retard side hydraulic chamber.
Operation of the variable valve timing mechanism involves adjusting the hydraulic pressures in the advance side hydraulic chamber and the retard side hydraulic chamber to set the overlap amount of the intake valve and the exhaust valve large when the engine is operating at high speed and small when the engine is operating at low speed.
Also, JP-A-2006-138292 states that the overlap amount is set extremely small when the engine is idling. Also, JP-A-2004-332662 states that the exhaust valve and the intake valve are controlled so that there is a negative overlap in which both the exhaust valve and the intake valve are closed from the exhaust stroke through the intake stroke when the engine is idling.
In JP-A-2006-138292 and JP-A-2004-332662 described above, regardless of the running state of the vehicle, when the engine is idling, the variable valve timing mechanism sets the overlap amount extremely small (JP-A-2006-138292), or controls the overlap to a negative overlap (JP-A-2004-332662). Therefore, for example, when the driver is not depressing the accelerator such that the accelerator operation amount is zero, and the vehicle speed is relatively low and the transmission is in a neutral state while the vehicle is traveling, the engine idles, so from this point, the overlap amount is set extremely small or is controlled to a negative overlap.
Incidentally, while the vehicle is traveling in this way, the driver may depress the accelerator, thus increasing the engine load. However, with the control in JP-A-2006-138292 and JP-A-2004-332662, the overlap amount is set extremely small or controlled to a negative overlap when the engine starts to idle, which makes it difficult to quickly obtain an overlap amount suitable for an increase in engine load. That is, the engine speed increases as the engine load increases while the vehicle is traveling, but the overlap amount is unable to be adjusted appropriately so the optimum overlap amount is unable to be quickly obtained.
One way to solve this problem is to keep the overlap amount large (i.e., to keep the overlap amount at an overlap amount that is suitable for a high engine speed) even if the engine is idling. However, if the vehicle is stopped after the driver releases the accelerator while the vehicle is still traveling, the overlap amount will be larger than the proper amount when the vehicle is stopped. As a result, vibration from the engine that is caused by the overlap amount being larger than the proper amount is transmitted to the vehicle cabin, which is unpleasant for occupants. (It is known that when the overlap amount is larger than the proper amount, combustion becomes unstable due to the effects of internal EGR and the like, and engine vibration tends to increase.) In particular, the majority of the vibration that is transmitted to the vehicle cabin while the vehicle is stopped is vibration from the engine, so a large amount of engine vibration is undesirable.